Group by minimum value in one field while selecting distinct rows

sql

prompt-engineering

best-practices

performance

byAlex Kataev·Jan 2, 2025

To group rows by the lowest value in a field while ensuring uniqueness, use a subquery to pinpoint these minimums, then join this result to pick out the corresponding records. Here's how you can do it using SQL:

SELECT main.*
FROM MyTable main
INNER JOIN (
    SELECT GroupColumn, MIN(ValueColumn) MinVal
    FROM MyTable
    GROUP BY GroupColumn
) sub ON main.GroupColumn = sub.GroupColumn AND main.ValueColumn = sub.MinVal

To simplify, this constructs a temporary result sub holding the smallest ValueColumn for each GroupColumn. We then join this to MyTable to fetch rows that hold the group-wise minimums.

Enhanced ways to tackle the problem

Single out records with ROW_NUMBER()

The ROW_NUMBER() window function offers a great way to handle this problem. Group the data by your GroupColumn and order it by ValueColumn (you can add more columns for tie-breaking).

-- The SQL equivalent of "You're number 1!"
SELECT *
FROM (
    SELECT *, ROW_NUMBER() OVER (PARTITION BY GroupColumn ORDER BY ValueColumn) as rn
    FROM MyTable
) sub
WHERE rn = 1 -- "I only care about Number 1."

Restrict your selection to rows where rn (row number) equals 1 to obtain distinct rows with the smallest ValueColumn for each GroupColumn.

Handling duplicate minimums

When you have multiple rows showcasing the minimum value for a group, a solid strategy is essential. Introducing more columns to the ROW_NUMBER() ordering, or opting for a DISTINCT ON clause can resolve any ambiguities.

Using subqueries for refined control

Correlated subqueries allow for a higher level of control over the minimum value selection, while also ensuring that every returned value is dynamic:

SELECT *
FROM MyTable main
WHERE ValueColumn = (
    SELECT MIN(ValueColumn)
    FROM MyTable sub
    WHERE main.GroupColumn = sub.GroupColumn -- "Oh, so you're of the same group. Let's talk."
)

Database compatibility

Tailor your SQL syntax to the database you're working with, whether it's MySQL, PostgreSQL, or something else. Certain clauses, like DISTINCT ON, are exclusive to some databases while MySQL would need a slightly different approach.

Performance considerations

On large datasets, using subqueries could potentially lead to slower performance. Proper indexing of columns involved in GROUP BY, ORDER BY, and JOIN conditions can drastically boost query performance.

Mastering complexities

Adding non-aggregated columns

Mindfully add non-aggregated columns to your results, making sure they are accounted for in your GROUP BY or through aggregate functions.

The advantage of the HAVING clause

Unlike WHERE, the HAVING clause allows for post-aggregation filtering, crucial for handling intricate criteria. Note that the filtering applies to the groups formed by GROUP BY, not individual rows.

Leveraging window function

The OVER() clause with ROW_NUMBER() works wonders for catering to various partitioning and ordering constraints, handling complex scenarios like managing ties.

explain-codes / Sql / Group by minimum value in one field while selecting distinct rows

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